Abstract: The intrinsic magnetic relaxation properties of goethite are elucidated via a comparative study of the magnetism of a stoichiometric and a nonstoichiometric sample, ?-FeOOH and ?-Fe0.920.08O0.76OH0.24, where denotes a vacancy site. It is proposed that both goethite samples exhibit ?mode superparamagnetism? at low temperatures, wherein the iron moments undergo thermally induced fluctuations over an energy barrier separating two closely related geometrical arrangements for the sublattice spins. This is an intrinsic relaxation mechanism that was previously predicted from magnetic structure refinements, which we have identified here in ac susceptibility and thermal decay of remanence data. An additional magnetic relaxation mode occurs in the nonstoichiometric goethite. This mode is characterized by relatively slow fluctuations, observable on the microsecond timescale of muon spin relaxation experiments, but not on the fast timescale of Mossbauer spectroscopy, and appearing in ac ¨susceptibility and dc magnetization data as blocking transitions in the range 70 to 100 K. These data are best described as the result of ?cluster ordering?: the formation and relaxation of magnetic clusters in the disturbed environment of the nonstoichiometric lattice.

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